Read channel apparatus and method for an optical storage system

ABSTRACT

A read channel apparatus is disclosed for reading data recorded on an optical storage system at a predetermined baud rate. The apparatus asynchronously samples an analog read signal generating from the optical storage system and subtracts an estimated DC offset from the asynchronous sample values to generate a sequence of asynchronous DC-removed sample values. The asynchronous DC-removed sample values are separately interpolated by two interpolators to generate a sequence of synchronous even-time sample values and a sequence of synchronous odd-time sample values respectively. The synchronous even-time and odd-time sample values are separately equalized by two equalizers in accordance with a target spectrum to generate a sequence of even-time equalized sample values and a sequence of odd-time equalized sample values respectively. A DC offset estimator generates the estimated DC offset from the even-time equalized sample values and the odd-time equalized sample values. The interpolators are under the control of a timing recovery controller for synchronizing the even-time and odd-time sample values to the baud rate. In the preferred embodiment, the recorded data are determined from the even-time equalized sample values and the odd-time equalized sample values.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/947,169, filed Sep. 5, 2001 now U.S. Pat. No. 6,904,084.

FIELD OF THE INVENTION

The present invention relates generally to an optical storage systemand, in particular, to a read channel apparatus and method for anoptical storage system.

BACKGROUND OF THE INVENTION

In recent years, the development of new optical recording media and datacompression techniques has made it possible to achieve enormous datastorage capacity using optical storage systems. Optical storage systemsare used to store audio information, such as in Compact Disk (CD)players, as well as visual and computer information, such as in CD-ROMand the more recent Digital Video Disk (DVD) players. The information istypically recorded as a binary sequence by writing a series of “pits” onthe optical medium which represent binary “1” and “0” bits. When readingthis recorded data, a pick-up head (transducer), positioned in closeproximity to the rotating disk, detects the alternations on the mediumand generates an analog read signal. The analog read signal is thendetected and decoded by read channel circuitry to reproduce the recordeddata.

To improve performance of the read channel in an optical storage system,the sampled amplitude techniques are applied. Sampled amplitude readchannels commonly employ an analog-to-digital converter (ADC) and adigital read channel processor to reproduce data recorded on the opticalstorage systems. However, in high-speed optical storage systems, thebaud rate (channel bit rate) is very high such that sampling frequencyof ADC and clock of digital processor also need comparable high clockrate sources. This is not desirable since operating the channel athigher frequencies increases its complexity and cost. There is,therefore, a need for a sampled amplitude read channel for use instorage systems that can operate at high data rates and densitieswithout increasing the cost and complexity of the read channel ICs. Tothis end, U.S. Pat. No. 5,802,118 (Bliss et al.) discloses a sub-sampleddiscrete time read channel for magnetic disk storage systems. Accordingto this patent, the read channel sub-samples an analog signal at a rateless than or equal to 9/10 the baud rate. K. C. Huang, the inventor ofpresent invention, discloses a sub-sampled method for read channel of anoptical storage system in Taiwan patent application No. 089,110,848,filed in June 2000. The prior art sub-samples an analog signal at a rateslightly above ½ the baud rate. The sub-sampled values are down-sampledby a timing recovery interpolator to generate sample values synchronizedto one-half the baud rate. The synchronous sample values are thenequalized by a 2T-spaced equalizer and interpolated by a factor-twoupsampler. Although it significantly reduces the sampling frequency, thelatency time introduced by the upsampler causes significant degradationin the performance of the high-speed optical storage systems.

For the reasons mentioned above, a novel read channel apparatus andmethod is provided to reproduce data recorded on the optical storagesystems, unencumbered by the limitations associated with the prior art.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a read channelapparatus and method for an optical storage system which sub-samples ananalog read signal at a rate slightly above one-half the baud rate toreduce clock rate of the read channel.

It is another object of the present invention to provide a read channelapparatus and method for an optical storage system that reproducesrecorded data with better performance caused by getting rid of thelatency of up-sampling.

To achieve the above object of the present invention, there is provideda read channel apparatus and method for reading data recorded on anoptical storage system at a predetermined baud rate. The read channelapparatus includes a sampling device, a subtractor, two interpolators,two equalizers, and a DC offset estimator. The sampling deviceasynchronously samples an analog read signal generating from the opticalstorage system to generate a sequence of asynchronous sample values. Thesubtractor subtracts an estimated DC offset from the asynchronous samplevalues to generate a sequence of asynchronous DC-removed sample values.Then, the first interpolator interpolates the asynchronous DC-removedsample values to generate a sequence of synchronous even-time samplevalues substantially synchronized to one-half the baud rate. The secondinterpolator also interpolates the asynchronous DC-removed sample valuesto generate a sequence of synchronous odd-time sample valuessubstantially synchronized to one-half the baud rate. Thereafter, thefirst equalizer equalizes the synchronous even-time sample values inaccordance with a target spectrum to generate a sequence of even-timeequalized sample values. The second equalizer equalizes the synchronousodd-time sample values in accordance with the target spectrum togenerate a sequence of odd-time equalized sample values. Besides, a DCoffset estimator generates the estimated DC offset from the even-timeequalized sample values and the odd-time equalized sample values. Thetwo interpolators are under the control of a timing recovery controllerfor synchronizing the even-time and odd-time sample values to the baudrate. In a preferred embodiment, a sequence detector detects therecorded data from the even-time equalized sample values and theodd-time equalized sample values.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and advantages of the present invention willbe better understood by reading the following detailed description ofthe invention in conjunction with the drawings, wherein:

FIG. 1 is a block diagram illustrating a read channel in accordance withthe present invention;

FIG. 2A shows the asynchronous sample values A_(K) sampled from ananalog read signal A(t);

FIG. 2B shows the synchronous even-time sample values X_(2K)substantially synchronized to ½ the baud rate;

FIG. 2C shows the synchronous odd-time sample values X_(2K+1)substantially synchronized to ½ the baud rate;

FIG. 2D shows the even-time equalized sample values Y_(2K);

FIG. 2E shows the odd-time equalized sample values Y_(2K+1);

FIG. 2F shows a sequence Y_(K) composed of Y_(2K) and Y_(2K+1) whereinall sample values are substantially synchronized to the baud rate; and

FIG. 2G shows a binary sequence Z_(K) determined by a sequence detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIGS. 1 and 2A˜2G, the present invention will becomemore apparent from the following detailed description. A read channelapparatus 200 for reading data recorded on an optical storage system ata predetermined baud rate is shown in FIG. 1. The analog read signalA(t), generating from the optical storage system, is asynchronouslysampled by a sampling device 202 (e.g., an analog-to-digital converter)to generate a sequence of asynchronous sample values A_(K). The samplingdevice 202 is clocked at a constant frequency fs by a sampling clock 240generated by a clock source 220. The sequence of asynchronous samplevalues A_(K) is related to the analog read signal A(t) asA _(K) ={ . . . , A(0), A(τ), A(2τ), . . . , A(kτ), . . . }where k is an integer and τ=1/fs. In FIG. 2A, the analog read signal isshown as a solid line, and the asynchronous sample values A_(K) areshown as black dots. It should be noted that T, as depicted in FIGS.2A˜2G, denotes the baud rate interval and 1/T denotes the baud rate.Since the frequency spectrum of the analog read channel A(t) receivedfrom the read transducer is bandlimited to about ¼T. Thus, the samplingdevice 202 only needs to sample the analog read signal A(t) at a rateslight above ½ the baud rate, i.e., fs>½T, rather than synchronoussampling at the baud rate. In FIG. 2A, for instance, the analog readsignal A(t) is sampled at 4/7 the baud rate.

With continued reference to FIG. 1, a subtractor 204 subtracts anestimated DC offset, over line 230, from the asynchronous sample valuesA_(K) to generate a sequence of asynchronous DC-removed sample valuesB_(K). This can be denoted asB _(K) ={ . . . , B(0), B(τ), B(2τ) . . . , B(kτ), . . . }andB(kτ)=A(kτ)−DCwhere DC denotes the estimated DC offset.

According to the present invention, the first interpolator 206 ainterpolates the asynchronous DC-removed sample values B_(K) andgenerates a sequence of synchronous even-time sample values X_(2K)substantially synchronized to one-half the baud rate. The secondinterpolator 206 b also interpolates the asynchronous DC-removed samplevalues B_(K) to generate a sequence of synchronous odd-time samplevalues X_(2K+1) substantially synchronized to one-half the baud rate. Asdepicted in FIGS. 2B and 2C, the synchronous even-time sample valueX_(2K) is shown as “Δ” and the synchronous odd-time sample valueX_(2K+1) is shown as “x”. The synchronous even-time and odd-timesequences can be denoted asX _(2K) ={ . . . , X(0), X(2T), X(4T), . . . , X(2kT), . . . }andX _(2K+1) ={ . . . , X(T), X(3T), X(5T), . . . , X((2k+1)T), . . . }where k is an integer and T is the baud rate interval. Hence, thesynchronous even-time sequence X_(2K) has a spacing between samplesequal to 2T and the synchronous odd-time sequence X_(2K+1) also has aspacing between samples equal to 2T. For more details concerning theinterpolators 206 a and 206 b, refer to F. M. Gardner, “Interpolation inDigital Modems—Part I: Fundamentals”, IEEE Trans. Commun., Vol. 41, pp.502–508, March 1993; and L. Erup, F. M. Gardner, and R. A. Herris,“Interpolation in Digital Modems—Part II: Implementation andperformance”, IEEE Trans. Commun., Vol. 41, pp. 998–1008, June 1993.

The synchronous even-time and odd-time sequences are separatelyequalized according to a target spectrum. That is, the high-frequencycomponents of X_(2K) and X_(2K+1) are enhanced by equalization. This canbe implemented with, for instance, two 5-tap symmetric 2-T spacedequalizers. A first 2T-spaced equalizer 208 a equalizes the synchronouseven-time sample values X_(2K) in accordance with the target spectrum togenerate a sequence of even-time equalized sample values Y_(2K). Thesecond 2T-spaced equalizer 208 b equalizes the synchronous odd-timesample values X_(2K+1) in accordance with the target spectrum togenerate a sequence of odd-time equalized sample values Y_(2K+1). Thefirst and second 2T-spaced equalizers preferably employ the samecoefficients. Referring to FIGS. 2D and 2E, the even-time and odd-timeequalized sequences can be represented byY _(2K) ={ . . . , Y(0), Y(2T), Y(4T), . . . , Y(2kT), . . . }andY _(2K+1) ={ . . . , Y(T), Y(3T), Y(5T), . . . , Y((2k+1)T), . . . }

The even-time and odd-time equalized sequences are fed back to a timingrecovery controller 210 and a DC offset estimator 212. The timingrecovery controller 210 respectively controls the first interpolator 206a and the second interpolator 206 b, in response to Y_(2K) and Y_(2K+1)received over line 226 and line 228, to synchronize the even-time andodd-time sample values to the baud rate. The DC offset estimator 212, inresponse to Y_(2K) and Y_(2K+1) received over line 232 and line 234,generates the estimated DC offset.

Referring to FIGS. 2F and 2G, if the quality (e.g., signal-to-noiseratio) of the analog read signal is good enough, an estimated binarysequence Z_(K) representing recorded data can be determined directlyfrom Y_(K). The sequence Y_(K) is composed of Y_(2K) and Y_(2K+1) asillustrated in FIG. 2F. For example, the m-th bit of Z_(K) is estimatedto be “1” if Y(mT)>0, and “0” if y(mT)<0. However, if the quality of theanalog read signal is poor, a sequence detector 214 is preferablyapplied to detect the sequence Z_(K) from Y_(2K) and Y_(2K+1) Thesequence detector 214 typically utilizes the Viterbi algorithm toimplement the run length limitation of DVD systems. In DVD, the minimumrun length is 3, that is, the sequence with “ . . . 0000100 . . . ”, “ .. . 00011000 . . . ”, “ . . . 1110111 . . . ”, and “ . . . 11100111 . .. ” are not allowed in DVD and will be filtered out by the sequencedetector 214. Thus, the read channel apparatus 200 as described above,processes the even-time and odd-time sample values separately so that itavoids degrading in performance caused by an up-sampling latency.

The objects of the invention have been fully realized through thepreferred embodiment disclosed herein. It will be apparent that theinvention is not limited thereto, and that many modifications andadditions may be made within the scope of the invention. Therefore, itis the object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

1. A read channel apparatus for reading data recorded on an opticalstorage system at a predetermined baud rate, comprising: a samplingdevice for asynchronously sampling an analog read signal generating fromthe optical storage system to generate a sequence of asynchronous samplevalues; a subtractor for subtracting an estimated DC offset from theasynchronous sample values to generate a sequence of asynchronousDC-removed sample values; a first interpolator for interpolating theasynchronous DC-removed sample values to generate a sequence ofsynchronous even-time sample values substantially synchronized toone-half the baud rate; a second interpolator for interpolating theasynchronous DC-removed sample values to generate a sequence ofsynchronous odd-time sample values substantially synchronized toone-half the baud rate; a timing recovery controller, responsive to theeven-time sample values and the odd-time sample values, for controllingthe first interpolator and the second interpolator respectively in orderto synchronize the even-time and odd-time sample values to the baudrate; and a DC offset estimator for generating the estimated DC offsetfrom the even-time sample values and the odd-time sample values; whereinthe recorded data are determined from the even-time sample values andthe odd-time sample values.
 2. The read channel apparatus of claim 1further comprising a sequence detector for detecting the recorded datafrom the even-time sample values and the odd-time sample values.
 3. Theread channel apparatus of claim 2, wherein the sequence detectorcomprises a Viterbi sequence detector.
 4. The read channel apparatus ofclaim 1, wherein the sampling device samples the analog read signal at asampling rate slightly above ½ the baud rate of the data recorded on theoptical storage system.
 5. The read channel apparatus of claim 4,wherein the sampling device comprises an analog-to-digital converter. 6.A read channel method for reading data recorded on an optical storagesystem at a predetermined baud rate, comprising: asynchronously samplingan analog read signal from the optical storage system to generate asequence of asynchronous sample values; subtracting an estimated DCoffset from the asynchronous sample values to generate a sequence ofasynchronous DC-removed sample values; separately interpolating theasynchronous DC-removed sample values to respectively generate asequence of synchronous even-time sample values substantiallysynchronized to one-half the baud rate and a sequence of synchronousodd-time sample values substantially synchronized to one-half the baudrate; generating the estimated DC offset from the even-time samplevalues and the odd-time sample values; and detecting the recorded datafrom the even-time sample values and the odd-time sample values.
 7. Theread channel method of claim 6, wherein the detecting step performs arun length limitation with Viterbi detection to determine the recordeddata from the even-time sample values and the odd-time sample values. 8.The read channel method of claim 6, wherein the sampling step samplesthe analog read signal at a sampling rate slightly above ½ the baud rateof the data recorded on the optical storage system.
 9. A read channelapparatus for reading data recorded on an optical storage system at apredetermined baud rate, comprising: a sampling device forasynchronously sampling an analog read signal generating from theoptical storage system to generate a sequence of asynchronous samplevalues; a subtractor for subtracting an estimated DC offset from theasynchronous sample values to generate a sequence of asynchronousDC-removed sample values; N interpolators for interpolating theasynchronous DC-removed sample values to generate N sets of synchronoussample values, wherein the N sets of synchronous sample values form asequence of synchronous sample values substantially synchronized to thebaud rate; a timing recovery controller, responsive to the N sets ofsynchronous sample values, for controlling the N interpolatorsrespectively in order to synchronize the N sets of synchronous samplevalues to the baud rate; and a DC offset estimator for generating theestimated DC offset from the N sets of synchronous sample values;wherein the recorded data are determined from the N sets of synchronoussample values.
 10. The read channel apparatus of claim 9, furthercomprising a sequence detector for detecting the recorded data from theN sets of synchronous sample values.
 11. The read channel apparatus ofclaim 9, further comprising an equalizer equalizing the N sets ofsynchronous sample values in accordance with a target spectrum togenerate N sets of equalized sample values.
 12. The read channelapparatus of claim 11, further comprising a sequence detector fordetecting the recorded data from the N sets of equalized sample values.13. The read channel apparatus of claim 12, wherein the sequencedetector comprises a Viterbi sequence detector.
 14. The read channelapparatus of claim 9, wherein the sampling device samples the analogread signal at a sampling rate slightly above ½ the baud rate of thedata recorded on the optical storage system.
 15. The read channelapparatus of claim 14, wherein the sampling device comprises ananalog-to-digital converter.
 16. A read channel method for reading datarecorded on an optical storage system at a predetermined baud rate,comprising: asynchronously sampling an analog read signal from theoptical storage system to generate a sequence of asynchronous samplevalues; subtracting an estimated DC offset from the asynchronous samplevalues to generate a sequence of asynchronous DC-removed sample values;separately interpolating the asynchronous DC-removed sample values torespectively generate N sets of synchronous sample values, wherein the Nsets of synchronous sample values form a sequence of synchronous samplevalues substantially synchronized to the baud rate; generating theestimated DC offset from the synchronous sample values; and detectingthe recorded data from the synchronous sample values.
 17. The readchannel method of claim 16, wherein the detecting step performs a runlength limitation with Viterbi detection to determine the recorded datafrom the synchronous sample values.
 18. The read channel method of claim16, wherein the sampling step samples the analog read signal at asampling rate slightly above ½ the baud rate of the data recorded on theoptical storage system.
 19. The read channel method of claim 16, furthercomprising equalizing the N sets of synchronous sample values inaccordance with a target spectrum to generate N sets of equalized samplevalues, and detecting the recorded data from the N sets of equalizedsample values.